• Direct exchange between every cell and the environment is not possible in all animals
  • special structures for exchange & transport needed

• Respiratory systems:

• Circulatory systems:

• Internal transport and gas exchange functionally related

• Small molecules can move between cells and their surroundings by diffusion

• Diffusion is only efficient over small distances

• Molecular trading in animals occurs for every cell!
  • gain O₂ and nutrients
  • release CO₂ and waste products

• Simple body plans:
• Complex body plans:

• Many animals have lots of cells in contact with environment
  • hydras, jellyfish, flatworms

• Central gastrovascular cavity functions to distribute substances
  • opening at one end connects cavity to water
  • allows fluid to bath inner and outer tissues
  • diffusion distance kept short

• Gastrovascular cavity can be highly branched
  • extends into tentacles of cnidarians
  • covers high surface area of flatworms

• Circulatory systems have 3 components

• Fluid transport, via vessels, connects cytoplasm of cells to organs for gas exchange

• Mammals → O₂ diffuses over 2 cell layers in lungs to reach blood
  • molecule trading works like a circuit

• Cardiovascular system: blood → heart → vessels
  • length of vessels can be staggering

• 3 main types of vessels

1. Arteries:
2. Veins:
3. Capillaries:

• Hearts of vertebrates have 2+ muscular chambers
  • atria:
  • ventricles:

• Number of chambers related to form and function

• Amphibians and reptile hearts have 1 ventricle
  • still use double circulation

• Do not always fill their lungs
  • pass long periods without gas exchange
  • or use another tissue (skin)

• Example: frogs use skin when underwater
• Example: crocodiles shunt blood from lungs when underwater

• The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle

• Right ventricle pumps blood to the lungs via arteries

• Blood flows through capillaries into lungs and loads O₂ and unloads CO₂

• O₂ rich blood returns from lungs via veins to the left atrium

• Blood flows to left ventricle and pumped to tissues via arteries into capillaries
  • First branches supply the heart muscle
  • Further branches lead to organs and hind limbs

• O₂ diffuses from blood to tissues, and CO₂ diffuses from tissues to blood

• Capillaries rejoin conveying blood to veins
  • O₂ poor blood from the head, neck, and forelimbs is emptied into right atrium

• Physical laws govern movement of fluids through pipes
  • affect blood flow and pressure

• Blood flows from areas of higher pressure to low
  • narrow diameter capillaries decrease pressure

• Blood flow is slowest in the capillaries
  • over large cross-sectional area
  • necessary for gas exchange

• Gas exchange = uptake of O₂ and release of CO₂
  • O₂ is abundant in the air (21%)
  • fairly easily to ‘breath’

• Water is more demanding for gas exchange
  • dissolved O₂ is variable but always less than air

• Efficient surfaces for gas exchange have evolved

• Differs across animal phylogeny
  • sponges, cnidaria, flatworm → cells directly
  • earthworms, amphibians → skin

• Other animals lack enough surface area
  • to much volume to supply with O₂

• Respiratory organs evolve
  • Gills → aquatic animals
  • Trachea → insects
  • Lungs → reptiles and mammals

• Most animals need to exchange large amounts of O₂ and CO₂
  • to meet metabolic needs

• During inhalation, fresh air fills in lung
  • molecule trading occurs via diffusion

• Blood is then pumped through circuit
  • O₂ carried in pigments (metal bound to protein)